Transcript Slide 1

Secondary electron detectors
Anderson Thesing – January, 2013
Principle of the SEM
The SEM is an instrument
that scans a sample
surface with a finely
converged electron beam
in a vacuum, detects the
signals produced at that
time from the sample,
and presents a image of
the surface of the
sample.
By irradiating the sample with an electron beam in a
vacuum, secondary electrons, backscattered electrons, xrays and other signals are generated. For the SEM image,
secondary electrons and backscattered electrons are
used.
Secundary electrons (SE)
• SE are produced near to surface of the sample, and the
SE image obtained upon detecting these electrons
reflects the fine topographical structure of the sample.
• SE have a very small energy emitted(≥50 eV), therefore,
a high resolution image is expectable from the SE signals.
There are 3 types of SE:
SE1: First kind of secondary electrons are generated
by the incoming electron beam as they enter the
surface. The resulted image has high resolution
which is only limited by the electron beam
diameter.
SE2: The secondary electrons that are generated by the
backscattered electrons have returned to the surface after
several inelastic scattering events. SE2 come from a surface
area which is bigger than the spot of the incoming electrons ,
also bigger than surface area of SE1. Therefore resolution is
less than resolution of SE1.
SE3: The generator of
this kind is the SE2 that
enters the lens surface and get scattered. Thus, it
will result in a decrease of resolution.
Factors that affect the SE emission
• Atomic number (Z): when the Z increase, the emission
of SE increase too.
• Incident angle of electron beam
Factors that affect the SE emission
• Energy beam: the maximum emission of the SE
depends of the specimen.
𝑁𝑆𝐸
• Electron emission yield 𝛿 =
𝑁𝐼𝐸
Everhart and Thornley detector
• Long service life
• Fast response speed
In summary: A SEM image generally refers to a
secondary electron image. This image is indispensable
particularly for high resolution observation, based on the
information source of secondary electron is shallow and
small.
Thank you!